nbd/client.c

   1 /*
   2  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
   3  *
   4  *  Network Block Device Client Side
   5  *
   6  *  This program is free software; you can redistribute it and/or modify
   7  *  it under the terms of the GNU General Public License as published by
   8  *  the Free Software Foundation; under version 2 of the License.
   9  *
  10  *  This program is distributed in the hope that it will be useful,
  11  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13  *  GNU General Public License for more details.
  14  *
  15  *  You should have received a copy of the GNU General Public License
  16  *  along with this program; if not, see <http://www.gnu.org/licenses/>.
  17  */
  18
  19 #include "qemu/osdep.h"
  20 #include "qapi/error.h"
  21 #include "nbd-internal.h"
  22
  23 static int nbd_errno_to_system_errno(int err)
  24 {
  25     switch (err) {
  26     case NBD_SUCCESS:
  27         return 0;
  28     case NBD_EPERM:
  29         return EPERM;
  30     case NBD_EIO:
  31         return EIO;
  32     case NBD_ENOMEM:
  33         return ENOMEM;
  34     case NBD_ENOSPC:
  35         return ENOSPC;
  36     case NBD_EINVAL:
  37     default:
  38         return EINVAL;
  39     }
  40 }
  41
  42 /* Definitions for opaque data types */
  43
  44 static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
  45
  46 /* That's all folks */
  47
  48 /* Basic flow for negotiation
  49
  50    Server         Client
  51    Negotiate
  52
  53    or
  54
  55    Server         Client
  56    Negotiate #1
  57                   Option
  58    Negotiate #2
  59
  60    ----
  61
  62    followed by
  63
  64    Server         Client
  65                   Request
  66    Response
  67                   Request
  68    Response
  69                   ...
  70    ...
  71                   Request (type == 2)
  72
  73 */
  74
  75
  76 /* If type represents success, return 1 without further action.
  77  * If type represents an error reply, consume the rest of the packet on ioc.
  78  * Then return 0 for unsupported (so the client can fall back to
  79  * other approaches), or -1 with errp set for other errors.
  80  */
  81 static int nbd_handle_reply_err(QIOChannel *ioc, uint32_t opt, uint32_t type,
  82                                 Error **errp)
  83 {
  84     uint32_t len;
  85     char *msg = NULL;
  86     int result = -1;
  87
  88     if (!(type & (1 << 31))) {
  89         return 1;
  90     }
  91
  92     if (read_sync(ioc, &len, sizeof(len)) != sizeof(len)) {
  93         error_setg(errp, "failed to read option length");
  94         return -1;
  95     }
  96     len = be32_to_cpu(len);
  97     if (len) {
  98         if (len > NBD_MAX_BUFFER_SIZE) {
  99             error_setg(errp, "server's error message is too long");
 100             goto cleanup;
 101         }
 102         msg = g_malloc(len + 1);
 103         if (read_sync(ioc, msg, len) != len) {
 104             error_setg(errp, "failed to read option error message");
 105             goto cleanup;
 106         }
 107         msg[len] = '\0';
 108     }
 109
 110     switch (type) {
 111     case NBD_REP_ERR_UNSUP:
 112         TRACE("server doesn't understand request %d, attempting fallback",
 113               opt);
 114         result = 0;
 115         goto cleanup;
 116
 117     case NBD_REP_ERR_POLICY:
 118         error_setg(errp, "Denied by server for option %x", opt);
 119         break;
 120
 121     case NBD_REP_ERR_INVALID:
 122         error_setg(errp, "Invalid data length for option %x", opt);
 123         break;
 124
 125     case NBD_REP_ERR_TLS_REQD:
 126         error_setg(errp, "TLS negotiation required before option %x", opt);
 127         break;
 128
 129     default:
 130         error_setg(errp, "Unknown error code when asking for option %x", opt);
 131         break;
 132     }
 133
 134     if (msg) {
 135         error_append_hint(errp, "%s\n", msg);
 136     }
 137
 138  cleanup:
 139     g_free(msg);
 140     return result;
 141 }
 142
 143 static int nbd_receive_list(QIOChannel *ioc, char **name, Error **errp)
 144 {
 145     uint64_t magic;
 146     uint32_t opt;
 147     uint32_t type;
 148     uint32_t len;
 149     uint32_t namelen;
 150     int error;
 151
 152     *name = NULL;
 153     if (read_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 154         error_setg(errp, "failed to read list option magic");
 155         return -1;
 156     }
 157     magic = be64_to_cpu(magic);
 158     if (magic != NBD_REP_MAGIC) {
 159         error_setg(errp, "Unexpected option list magic");
 160         return -1;
 161     }
 162     if (read_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 163         error_setg(errp, "failed to read list option");
 164         return -1;
 165     }
 166     opt = be32_to_cpu(opt);
 167     if (opt != NBD_OPT_LIST) {
 168         error_setg(errp, "Unexpected option type %x expected %x",
 169                    opt, NBD_OPT_LIST);
 170         return -1;
 171     }
 172
 173     if (read_sync(ioc, &type, sizeof(type)) != sizeof(type)) {
 174         error_setg(errp, "failed to read list option type");
 175         return -1;
 176     }
 177     type = be32_to_cpu(type);
 178     error = nbd_handle_reply_err(ioc, opt, type, errp);
 179     if (error <= 0) {
 180         return error;
 181     }
 182
 183     if (read_sync(ioc, &len, sizeof(len)) != sizeof(len)) {
 184         error_setg(errp, "failed to read option length");
 185         return -1;
 186     }
 187     len = be32_to_cpu(len);
 188
 189     if (type == NBD_REP_ACK) {
 190         if (len != 0) {
 191             error_setg(errp, "length too long for option end");
 192             return -1;
 193         }
 194     } else if (type == NBD_REP_SERVER) {
 195         if (len < sizeof(namelen) || len > NBD_MAX_BUFFER_SIZE) {
 196             error_setg(errp, "incorrect option length");
 197             return -1;
 198         }
 199         if (read_sync(ioc, &namelen, sizeof(namelen)) != sizeof(namelen)) {
 200             error_setg(errp, "failed to read option name length");
 201             return -1;
 202         }
 203         namelen = be32_to_cpu(namelen);
 204         len -= sizeof(namelen);
 205         if (len < namelen) {
 206             error_setg(errp, "incorrect option name length");
 207             return -1;
 208         }
 209         if (namelen > 255) {
 210             error_setg(errp, "export name length too long %d", namelen);
 211             return -1;
 212         }
 213
 214         *name = g_new0(char, namelen + 1);
 215         if (read_sync(ioc, *name, namelen) != namelen) {
 216             error_setg(errp, "failed to read export name");
 217             g_free(*name);
 218             *name = NULL;
 219             return -1;
 220         }
 221         (*name)[namelen] = '\0';
 222         len -= namelen;
 223         if (len) {
 224             char *buf = g_malloc(len + 1);
 225             if (read_sync(ioc, buf, len) != len) {
 226                 error_setg(errp, "failed to read export description");
 227                 g_free(*name);
 228                 g_free(buf);
 229                 *name = NULL;
 230                 return -1;
 231             }
 232             buf[len] = '\0';
 233             TRACE("Ignoring export description: %s", buf);
 234             g_free(buf);
 235         }
 236     } else {
 237         error_setg(errp, "Unexpected reply type %x expected %x",
 238                    type, NBD_REP_SERVER);
 239         return -1;
 240     }
 241     return 1;
 242 }
 243
 244
 245 static int nbd_receive_query_exports(QIOChannel *ioc,
 246                                      const char *wantname,
 247                                      Error **errp)
 248 {
 249     uint64_t magic = cpu_to_be64(NBD_OPTS_MAGIC);
 250     uint32_t opt = cpu_to_be32(NBD_OPT_LIST);
 251     uint32_t length = 0;
 252     bool foundExport = false;
 253
 254     TRACE("Querying export list");
 255     if (write_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 256         error_setg(errp, "Failed to send list option magic");
 257         return -1;
 258     }
 259
 260     if (write_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 261         error_setg(errp, "Failed to send list option number");
 262         return -1;
 263     }
 264
 265     if (write_sync(ioc, &length, sizeof(length)) != sizeof(length)) {
 266         error_setg(errp, "Failed to send list option length");
 267         return -1;
 268     }
 269
 270     TRACE("Reading available export names");
 271     while (1) {
 272         char *name = NULL;
 273         int ret = nbd_receive_list(ioc, &name, errp);
 274
 275         if (ret < 0) {
 276             g_free(name);
 277             name = NULL;
 278             return -1;
 279         }
 280         if (ret == 0) {
 281             /* Server doesn't support export listing, so
 282              * we will just assume an export with our
 283              * wanted name exists */
 284             foundExport = true;
 285             break;
 286         }
 287         if (name == NULL) {
 288             TRACE("End of export name list");
 289             break;
 290         }
 291         if (g_str_equal(name, wantname)) {
 292             foundExport = true;
 293             TRACE("Found desired export name '%s'", name);
 294         } else {
 295             TRACE("Ignored export name '%s'", name);
 296         }
 297         g_free(name);
 298     }
 299
 300     if (!foundExport) {
 301         error_setg(errp, "No export with name '%s' available", wantname);
 302         return -1;
 303     }
 304
 305     return 0;
 306 }
 307
 308 static QIOChannel *nbd_receive_starttls(QIOChannel *ioc,
 309                                         QCryptoTLSCreds *tlscreds,
 310                                         const char *hostname, Error **errp)
 311 {
 312     uint64_t magic = cpu_to_be64(NBD_OPTS_MAGIC);
 313     uint32_t opt = cpu_to_be32(NBD_OPT_STARTTLS);
 314     uint32_t length = 0;
 315     uint32_t type;
 316     QIOChannelTLS *tioc;
 317     struct NBDTLSHandshakeData data = { 0 };
 318
 319     TRACE("Requesting TLS from server");
 320     if (write_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 321         error_setg(errp, "Failed to send option magic");
 322         return NULL;
 323     }
 324
 325     if (write_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 326         error_setg(errp, "Failed to send option number");
 327         return NULL;
 328     }
 329
 330     if (write_sync(ioc, &length, sizeof(length)) != sizeof(length)) {
 331         error_setg(errp, "Failed to send option length");
 332         return NULL;
 333     }
 334
 335     TRACE("Getting TLS reply from server1");
 336     if (read_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 337         error_setg(errp, "failed to read option magic");
 338         return NULL;
 339     }
 340     magic = be64_to_cpu(magic);
 341     if (magic != NBD_REP_MAGIC) {
 342         error_setg(errp, "Unexpected option magic");
 343         return NULL;
 344     }
 345     TRACE("Getting TLS reply from server2");
 346     if (read_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 347         error_setg(errp, "failed to read option");
 348         return NULL;
 349     }
 350     opt = be32_to_cpu(opt);
 351     if (opt != NBD_OPT_STARTTLS) {
 352         error_setg(errp, "Unexpected option type %x expected %x",
 353                    opt, NBD_OPT_STARTTLS);
 354         return NULL;
 355     }
 356
 357     TRACE("Getting TLS reply from server");
 358     if (read_sync(ioc, &type, sizeof(type)) != sizeof(type)) {
 359         error_setg(errp, "failed to read option type");
 360         return NULL;
 361     }
 362     type = be32_to_cpu(type);
 363     if (type != NBD_REP_ACK) {
 364         error_setg(errp, "Server rejected request to start TLS %x",
 365                    type);
 366         return NULL;
 367     }
 368
 369     TRACE("Getting TLS reply from server");
 370     if (read_sync(ioc, &length, sizeof(length)) != sizeof(length)) {
 371         error_setg(errp, "failed to read option length");
 372         return NULL;
 373     }
 374     length = be32_to_cpu(length);
 375     if (length != 0) {
 376         error_setg(errp, "Start TLS reponse was not zero %x",
 377                    length);
 378         return NULL;
 379     }
 380
 381     TRACE("TLS request approved, setting up TLS");
 382     tioc = qio_channel_tls_new_client(ioc, tlscreds, hostname, errp);
 383     if (!tioc) {
 384         return NULL;
 385     }
 386     data.loop = g_main_loop_new(g_main_context_default(), FALSE);
 387     TRACE("Starting TLS hanshake");
 388     qio_channel_tls_handshake(tioc,
 389                               nbd_tls_handshake,
 390                               &data,
 391                               NULL);
 392
 393     if (!data.complete) {
 394         g_main_loop_run(data.loop);
 395     }
 396     g_main_loop_unref(data.loop);
 397     if (data.error) {
 398         error_propagate(errp, data.error);
 399         object_unref(OBJECT(tioc));
 400         return NULL;
 401     }
 402
 403     return QIO_CHANNEL(tioc);
 404 }
 405
 406
 407 int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint32_t *flags,
 408                           QCryptoTLSCreds *tlscreds, const char *hostname,
 409                           QIOChannel **outioc,
 410                           off_t *size, Error **errp)
 411 {
 412     char buf[256];
 413     uint64_t magic, s;
 414     int rc;
 415
 416     TRACE("Receiving negotiation tlscreds=%p hostname=%s.",
 417           tlscreds, hostname ? hostname : "<null>");
 418
 419     rc = -EINVAL;
 420
 421     if (outioc) {
 422         *outioc = NULL;
 423     }
 424     if (tlscreds && !outioc) {
 425         error_setg(errp, "Output I/O channel required for TLS");
 426         goto fail;
 427     }
 428
 429     if (read_sync(ioc, buf, 8) != 8) {
 430         error_setg(errp, "Failed to read data");
 431         goto fail;
 432     }
 433
 434     buf[8] = '\0';
 435     if (strlen(buf) == 0) {
 436         error_setg(errp, "Server connection closed unexpectedly");
 437         goto fail;
 438     }
 439
 440     TRACE("Magic is %c%c%c%c%c%c%c%c",
 441           qemu_isprint(buf[0]) ? buf[0] : '.',
 442           qemu_isprint(buf[1]) ? buf[1] : '.',
 443           qemu_isprint(buf[2]) ? buf[2] : '.',
 444           qemu_isprint(buf[3]) ? buf[3] : '.',
 445           qemu_isprint(buf[4]) ? buf[4] : '.',
 446           qemu_isprint(buf[5]) ? buf[5] : '.',
 447           qemu_isprint(buf[6]) ? buf[6] : '.',
 448           qemu_isprint(buf[7]) ? buf[7] : '.');
 449
 450     if (memcmp(buf, "NBDMAGIC", 8) != 0) {
 451         error_setg(errp, "Invalid magic received");
 452         goto fail;
 453     }
 454
 455     if (read_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 456         error_setg(errp, "Failed to read magic");
 457         goto fail;
 458     }
 459     magic = be64_to_cpu(magic);
 460     TRACE("Magic is 0x%" PRIx64, magic);
 461
 462     if (magic == NBD_OPTS_MAGIC) {
 463         uint32_t clientflags = 0;
 464         uint32_t opt;
 465         uint32_t namesize;
 466         uint16_t globalflags;
 467         uint16_t exportflags;
 468         bool fixedNewStyle = false;
 469
 470         if (read_sync(ioc, &globalflags, sizeof(globalflags)) !=
 471             sizeof(globalflags)) {
 472             error_setg(errp, "Failed to read server flags");
 473             goto fail;
 474         }
 475         globalflags = be16_to_cpu(globalflags);
 476         *flags = globalflags << 16;
 477         TRACE("Global flags are %x", globalflags);
 478         if (globalflags & NBD_FLAG_FIXED_NEWSTYLE) {
 479             fixedNewStyle = true;
 480             TRACE("Server supports fixed new style");
 481             clientflags |= NBD_FLAG_C_FIXED_NEWSTYLE;
 482         }
 483         /* client requested flags */
 484         clientflags = cpu_to_be32(clientflags);
 485         if (write_sync(ioc, &clientflags, sizeof(clientflags)) !=
 486             sizeof(clientflags)) {
 487             error_setg(errp, "Failed to send clientflags field");
 488             goto fail;
 489         }
 490         if (tlscreds) {
 491             if (fixedNewStyle) {
 492                 *outioc = nbd_receive_starttls(ioc, tlscreds, hostname, errp);
 493                 if (!*outioc) {
 494                     goto fail;
 495                 }
 496                 ioc = *outioc;
 497             } else {
 498                 error_setg(errp, "Server does not support STARTTLS");
 499                 goto fail;
 500             }
 501         }
 502         if (!name) {
 503             TRACE("Using default NBD export name \"\"");
 504             name = "";
 505         }
 506         if (fixedNewStyle) {
 507             /* Check our desired export is present in the
 508              * server export list. Since NBD_OPT_EXPORT_NAME
 509              * cannot return an error message, running this
 510              * query gives us good error reporting if the
 511              * server required TLS
 512              */
 513             if (nbd_receive_query_exports(ioc, name, errp) < 0) {
 514                 goto fail;
 515             }
 516         }
 517         /* write the export name */
 518         magic = cpu_to_be64(magic);
 519         if (write_sync(ioc, &magic, sizeof(magic)) != sizeof(magic)) {
 520             error_setg(errp, "Failed to send export name magic");
 521             goto fail;
 522         }
 523         opt = cpu_to_be32(NBD_OPT_EXPORT_NAME);
 524         if (write_sync(ioc, &opt, sizeof(opt)) != sizeof(opt)) {
 525             error_setg(errp, "Failed to send export name option number");
 526             goto fail;
 527         }
 528         namesize = cpu_to_be32(strlen(name));
 529         if (write_sync(ioc, &namesize, sizeof(namesize)) !=
 530             sizeof(namesize)) {
 531             error_setg(errp, "Failed to send export name length");
 532             goto fail;
 533         }
 534         if (write_sync(ioc, (char *)name, strlen(name)) != strlen(name)) {
 535             error_setg(errp, "Failed to send export name");
 536             goto fail;
 537         }
 538
 539         if (read_sync(ioc, &s, sizeof(s)) != sizeof(s)) {
 540             error_setg(errp, "Failed to read export length");
 541             goto fail;
 542         }
 543         *size = be64_to_cpu(s);
 544         TRACE("Size is %" PRIu64, *size);
 545
 546         if (read_sync(ioc, &exportflags, sizeof(exportflags)) !=
 547             sizeof(exportflags)) {
 548             error_setg(errp, "Failed to read export flags");
 549             goto fail;
 550         }
 551         exportflags = be16_to_cpu(exportflags);
 552         *flags |= exportflags;
 553         TRACE("Export flags are %x", exportflags);
 554     } else if (magic == NBD_CLIENT_MAGIC) {
 555         if (name) {
 556             error_setg(errp, "Server does not support export names");
 557             goto fail;
 558         }
 559         if (tlscreds) {
 560             error_setg(errp, "Server does not support STARTTLS");
 561             goto fail;
 562         }
 563
 564         if (read_sync(ioc, &s, sizeof(s)) != sizeof(s)) {
 565             error_setg(errp, "Failed to read export length");
 566             goto fail;
 567         }
 568         *size = be64_to_cpu(s);
 569         TRACE("Size is %" PRIu64, *size);
 570
 571         if (read_sync(ioc, flags, sizeof(*flags)) != sizeof(*flags)) {
 572             error_setg(errp, "Failed to read export flags");
 573             goto fail;
 574         }
 575         *flags = be32_to_cpup(flags);
 576     } else {
 577         error_setg(errp, "Bad magic received");
 578         goto fail;
 579     }
 580
 581     if (read_sync(ioc, &buf, 124) != 124) {
 582         error_setg(errp, "Failed to read reserved block");
 583         goto fail;
 584     }
 585     rc = 0;
 586
 587 fail:
 588     return rc;
 589 }
 590
 591 #ifdef __linux__
 592 int nbd_init(int fd, QIOChannelSocket *sioc, uint32_t flags, off_t size)
 593 {
 594     TRACE("Setting NBD socket");
 595
 596     if (ioctl(fd, NBD_SET_SOCK, sioc->fd) < 0) {
 597         int serrno = errno;
 598         LOG("Failed to set NBD socket");
 599         return -serrno;
 600     }
 601
 602     TRACE("Setting block size to %lu", (unsigned long)BDRV_SECTOR_SIZE);
 603
 604     if (ioctl(fd, NBD_SET_BLKSIZE, (size_t)BDRV_SECTOR_SIZE) < 0) {
 605         int serrno = errno;
 606         LOG("Failed setting NBD block size");
 607         return -serrno;
 608     }
 609
 610     TRACE("Setting size to %zd block(s)", (size_t)(size / BDRV_SECTOR_SIZE));
 611
 612     if (ioctl(fd, NBD_SET_SIZE_BLOCKS, (size_t)(size / BDRV_SECTOR_SIZE)) < 0) {
 613         int serrno = errno;
 614         LOG("Failed setting size (in blocks)");
 615         return -serrno;
 616     }
 617
 618     if (ioctl(fd, NBD_SET_FLAGS, flags) < 0) {
 619         if (errno == ENOTTY) {
 620             int read_only = (flags & NBD_FLAG_READ_ONLY) != 0;
 621             TRACE("Setting readonly attribute");
 622
 623             if (ioctl(fd, BLKROSET, (unsigned long) &read_only) < 0) {
 624                 int serrno = errno;
 625                 LOG("Failed setting read-only attribute");
 626                 return -serrno;
 627             }
 628         } else {
 629             int serrno = errno;
 630             LOG("Failed setting flags");
 631             return -serrno;
 632         }
 633     }
 634
 635     TRACE("Negotiation ended");
 636
 637     return 0;
 638 }
 639
 640 int nbd_client(int fd)
 641 {
 642     int ret;
 643     int serrno;
 644
 645     TRACE("Doing NBD loop");
 646
 647     ret = ioctl(fd, NBD_DO_IT);
 648     if (ret < 0 && errno == EPIPE) {
 649         /* NBD_DO_IT normally returns EPIPE when someone has disconnected
 650          * the socket via NBD_DISCONNECT.  We do not want to return 1 in
 651          * that case.
 652          */
 653         ret = 0;
 654     }
 655     serrno = errno;
 656
 657     TRACE("NBD loop returned %d: %s", ret, strerror(serrno));
 658
 659     TRACE("Clearing NBD queue");
 660     ioctl(fd, NBD_CLEAR_QUE);
 661
 662     TRACE("Clearing NBD socket");
 663     ioctl(fd, NBD_CLEAR_SOCK);
 664
 665     errno = serrno;
 666     return ret;
 667 }
 668 #else
 669 int nbd_init(int fd, QIOChannelSocket *ioc, uint32_t flags, off_t size)
 670 {
 671     return -ENOTSUP;
 672 }
 673
 674 int nbd_client(int fd)
 675 {
 676     return -ENOTSUP;
 677 }
 678 #endif
 679
 680 ssize_t nbd_send_request(QIOChannel *ioc, struct nbd_request *request)
 681 {
 682     uint8_t buf[NBD_REQUEST_SIZE];
 683     ssize_t ret;
 684
 685     TRACE("Sending request to server: "
 686           "{ .from = %" PRIu64", .len = %u, .handle = %" PRIu64", .type=%i}",
 687           request->from, request->len, request->handle, request->type);
 688
 689     cpu_to_be32w((uint32_t*)buf, NBD_REQUEST_MAGIC);
 690     cpu_to_be32w((uint32_t*)(buf + 4), request->type);
 691     cpu_to_be64w((uint64_t*)(buf + 8), request->handle);
 692     cpu_to_be64w((uint64_t*)(buf + 16), request->from);
 693     cpu_to_be32w((uint32_t*)(buf + 24), request->len);
 694
 695     ret = write_sync(ioc, buf, sizeof(buf));
 696     if (ret < 0) {
 697         return ret;
 698     }
 699
 700     if (ret != sizeof(buf)) {
 701         LOG("writing to socket failed");
 702         return -EINVAL;
 703     }
 704     return 0;
 705 }
 706
 707 ssize_t nbd_receive_reply(QIOChannel *ioc, struct nbd_reply *reply)
 708 {
 709     uint8_t buf[NBD_REPLY_SIZE];
 710     uint32_t magic;
 711     ssize_t ret;
 712
 713     ret = read_sync(ioc, buf, sizeof(buf));
 714     if (ret < 0) {
 715         return ret;
 716     }
 717
 718     if (ret != sizeof(buf)) {
 719         LOG("read failed");
 720         return -EINVAL;
 721     }
 722
 723     /* Reply
 724        [ 0 ..  3]    magic   (NBD_REPLY_MAGIC)
 725        [ 4 ..  7]    error   (0 == no error)
 726        [ 7 .. 15]    handle
 727      */
 728
 729     magic = be32_to_cpup((uint32_t*)buf);
 730     reply->error  = be32_to_cpup((uint32_t*)(buf + 4));
 731     reply->handle = be64_to_cpup((uint64_t*)(buf + 8));
 732
 733     reply->error = nbd_errno_to_system_errno(reply->error);
 734
 735     TRACE("Got reply: "
 736           "{ magic = 0x%x, .error = %d, handle = %" PRIu64" }",
 737           magic, reply->error, reply->handle);
 738
 739     if (magic != NBD_REPLY_MAGIC) {
 740         LOG("invalid magic (got 0x%x)", magic);
 741         return -EINVAL;
 742     }
 743     return 0;
 744 }
 745